Optical fiber coating die with reduced wetted length

ABSTRACT

An optical fiber coating apparatus that provides increased gyre stability and reduced gyre strength, thereby providing a more reliable coating application process during fiber drawing includes a cone-only coating die having a conical entrance portion with a tapered wall angled at a half angle α, wherein 2°≤α≤25°, and a cone height L 1  less than 2.2 mm, and a cylindrical portion having an inner diameter of d 2 , wherein 0.1 mm≤d 2 ≤0.5 mm and a cylindrical height of L 2 , wherein 0.05 mm≤L 2 ≤1.25 mm; a guide die having an optical fiber exit, the guide die disposed adjacent the cone-only coating die such that a wetted length (L 5 ) between the optical fiber exit of the guide die and the entrance of the cone-only coating die is from 1 mm to 5 mm; and a holder for holding the cone-only coating die and the guide die in a fixed relationship defining a coating chamber between the guide die and the cone-only coating die, the coating chamber having an inner radius L 6  from the optical fiber axis to an inner wall of the holder that is from 3 mm to 10 mm.

This application is a divisional of U.S. patent application Ser. No.15/868,029, filed on Jan. 11, 2018, which claims the benefit of priorityunder 35 U.S.C. § 119 of U.S. Provisional Application Ser. No.62/449,700 filed on Jan. 24, 2017 the content of which is relied uponand incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

This disclosure pertains to optical fiber coating dies and methods ofcoating optical fiber.

BACKGROUND OF THE DISCLOSURE

Optical fibers, commonly used in telecommunications, are typicallycoated with one or more generally concentric polymeric coatings toprotect the optical fiber from damage, such as from abrasion ormoisture. These protective coatings, typically radiation curable (e.g.,UV-curable), are applied as the fiber is being drawn. The drawn fiber ispassed through one or more coating (or sizing) dies having a cylindricalland portion having a diameter greater than the diameter of the opticalfiber. A liquid curable coating composition disposed above thecylindrical land portion is entrained by the fiber and pulled throughthe cylindrical land portion. It is important that the coating ormultiple coatings are concentric with the fiber and have a uniformthickness (or diameter) along the length of the fiber. These attributescontribute to ease in splicing and connectorization of the fiber,thereby providing lower losses in an installed fiber application. Marketdemands place increasingly stringent tolerances on the diameter andconcentricity of optical fiber coatings.

SUMMARY OF THE DISCLOSURE

Cone-only coating die designs are disclosed which have wetted length L₅(defined as the distance between the exit of the guide die and entranceof the cone-only coating die) such that 1 mm≤L₅≤5 mm, coating chamberinner radius L₆ such that 3 mm≤L₆≤10 mm, cone half angle between2°≤α≤25°, cone height L₁ between 0.25 mm and 2 mm; and a cylindricalland portion having an inner diameter of d₂ such that 0.1 mm≤d₂≤0.5 mm,and length L₂ such that 0.05 mm≤L₂≤1.25 mm. Such a die design results ina coating system having a smaller and more stable gyre that improves thestability of the coating application process.

As fiber draw speeds increase or the diameter of the sizing diedecreases, more coating is rejected in the sizing die taper, and thegyre in the sizing die bell gets stronger. This leads to both increasedinstability of the fiber position, which causes an increase in coatingoffset, and increased incidence of flooding, which causes fiber breaksand increased costs. Reducing the size of the gyre by reducing thewetted length also reduces the amount of coating rejected by the die.Reducing the strength and size of the gyre suppresses the random motionof the fiber, allowing the centering forces to improve the offset.Cone-only die designs disclosed herein with the described wetted lengthand coating chamber inner diameter characteristics result in increasedstability and reduced strength of the gyre, thereby resulting in a morestable and reliable coating application process in manufacturing,particularly for applications at draw speed≥50 mps (meters per second).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a fiber drawing and coating system used inthe production of optical fibers.

FIG. 2 is a schematic view of a conventional coating die having abell-shaped opening.

FIG. 3 shows the structure of a gyre in a conventional coating die.

FIG. 4 is a schematic view of a conventional wet-on-wet apparatus havingstacked guide and sizing dies.

FIG. 5 is a schematic view of a cone-only die coating system inaccordance with this disclosure.

FIGS. 6A-6D show size and structure of gyres for different wettedlengths.

FIGS. 7A-7D show the radial velocity profiles for gyres for thedifferent wetted lengths corresponding with FIGS. 6A-6D, respectively.

FIG. 8 is a graph showing the lubrication pressures for the fourdifferent wetted lengths of FIGS. 6A-6D and 7A-7D.

FIG. 9 is a graph showing the maximum temperatures for the fourdifferent wetted lengths of FIGS. 6A-6D and 7A-7D.

FIG. 10 is a graph showing the predicted coated diameters for the fourdifferent wetted lengths of FIGS. 6A-6D and 7A-7D.

FIG. 11 is a graph showing the predicted shear stresses for the fourdifferent wetted lengths of FIGS. 6A-6D and 7A-7D.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

A fiber drawing and coating system, used in the production of opticalfibers, is shown in FIG. 1. Fiber 10 is drawn from preform 11 which isheated in furnace 1. Fiber 10 passes through fiber cooling device 2 andthen through primary coater 3 where it is coated with a layer of primarycoating material. The primary coating layer is cured in primary coatingcuring device 4, and the diameter of the fiber including the curedprimary coating is measured by device 5. Curing device 4 typicallycomprises an irradiator array. Fiber 10 passes through secondary coater6 where it is coated with a layer of secondary coating material that iscured in curing device 7 which is similar to curing device 4. Thediameter of the fiber including the cured secondary coating is measured,for example by device 8. In some embodiments, fiber may also passthrough an optional tertiary ink coater device where it is coated with alayer of ink coating material that is cured in curing device similar tocuring devices 4 and 7. Tractor means 9 pulls the fiber 10 from furnace1 and through the intermediate devices. The drawn fiber is typicallytaken up onto spools by a winder (not shown) for further processing.Coating material is supplied to coaters 3 and 6 from sources 12 and 14,respectively. The inlet or delivery temperature of the coating materialcan be maintained at a desired value by devices 13 and 15, respectively,which are in communication with the coating delivery line.

Alternatively, the fiber passes through the primary coater 3 and thenthrough the secondary coater 6, without passing through a primarycoating curing device 4 in between the two coaters. The second coatingis applied directly on the primary coating before both coatings arecured. This is known as “wet-on-wet” or “WOW” application process.

FIG. 2 shows a conventional die 29 used to apply coating to opticalfiber. Dies with this design, which is at least 25 years old, are widelyavailable commercially from vendors such as Oberg, Sancliff and Nextrom.The die has a bell-shaped opening 30, at the base of which is a conicaltaper 32, followed by a straight land section 34. The coated diameter ofthe fiber is largely determined by the ratio of the fiber diameter tothe land diameter, although it is also affected by the fibertemperature, the length and slope of the taper, the distance from theguide die to the sizing die, the coating viscosity, and other parametersknown to those of skill in the art.

During operation, the fiber entrains a boundary layer of coating, mostof which is rejected in the bell and taper. The rejection causespressures of as much as 800 psi to build up at the apex of the tapercone, and this pressure forces additional coating through the die land.The pressure also creates a very large force that centers the fiber inthe die. The rejected coating forms a torus-shaped circulating cell,known as a vortex or gyre, in the region above the land, and this gyreis not stable. In FIG. 3 is shown the structure of the gyre 25 in aconventional coating die, with the gyre causing unstable behavior in thedie.

The coating temperature in the vortex is elevated compared to the bulktemperature, owing to shear heating resulting from the very high shearrates in the vortex. Finite-element/finite-volume computer modelsindicate that the coating temperature near the middle of the vortexincreases rapidly as the draw speed increases and may be as much as 80°C. higher than the coating feed temperature. The temperature increase isalso greater for smaller sizing dies, because more coating is rejected,and for greater drawing speeds, because the gyre spins faster. Becausethe vortex is not stable, some of this hot coating can escape via aneddy current or other perturbation, and when the hot coating enters thetaper region of the die, the centering forces are no longeraxisymmetric. The unbalanced forces again result in poor fiber-coatingoffset, i.e., the distance between the axial center of the coating andthe axial center of the optical fiber. Perhaps even more critically, thehot coating can migrate to the upper meniscus of the die. The lowerviscosity of the hot coating makes the process more susceptible toflooding, that is, to dewetting of the fiber and the consequent flowingof coating past the fiber upward through the entry chamber of the dieand out. When flooding occurs, the fiber typically breaks. Breaksrequire that the draw process be restarted, which can contributesignificantly to the manufacturing cost.

Another problem addressed herein is the poor offset observed when twocoatings are applied sequentially without curing, otherwise known aswet-on-wet application. A conventional wet-on-wet method is to stackstandard dies together including a guide die 40, a first coating die 41and a second coating die 42, as illustrated in FIG. 4.

The offset (distance between the centerline of the fiber and thecenterline of the coating) of both coatings is compromised with thismethod. The centering forces from the two dies are in competition witheach other, and these forces are random to some extent, as the coatinggyres in the two dies are chaotic and independent. Cone-only coatingdies have been reported (US 20150147467) to reduce size and strength ofcoating gyre. As referred to herein a “cone-only die” is a sizing diethat has a cone portion 52, but not the bell portion 30. A cone-only diehas a cylindrical land portion 54 and may have an optional exitcylindrical leg portion 55, as shown in FIG. 5, an optional uppercylindrical leg portion (not shown), or both an exit cylindrical legportion and an upper cylindrical leg portion. In the cone-only dieembodiment shown in FIG. 5, the bell portion 30 (formed by the convexwalls) of the standard die such as the one shown in FIG. 2 has beenremoved, and the cone region 52 and the cylindrical land region 54, aswell as optional upper cylindrical leg portion 55 are retained. The coneregion 52 preferably has a small height or length L₁ such L₁≤2.2 mm,preferably 0.2 mm≤L₁≤2 mm. For example, in some exemplary embodimentsthe height L₁ of the cone region 52 is 0.25 mm 0.3 mm, 0.4 mm, 0.5 mm,0.6 mm, 0.75 mm, 1 mm, 1.2 mm, 1.3 mm, 1.4 mm, 1.5 mm, 1.7 mm, 1.75 mm,1.8 mm, or there-between. More preferably, in order to reduce the sizeof the gyre, the height L₁ in certain embodiments is not greater than 2mm, even more preferably L₁≤1.8 mm, and most preferably L₁≤1.5 mm.Preferably L₁≥0.25 mm, more preferably L₁≥0.5 mm, and even preferablyL₁≥0.7 mm. According to some embodiments, 0.9 mm≤L₁≤1.2 mm.

Die designs are disclosed that include a conical ferrule with taperedwall, wherein the conical ferrule has a cross-section with inner wallsangled at a half angle α, where 2°≤α≤25°, and cone height L₁ between0.25 mm and 2 mm; and a cylindrical portion having an inner diameter ofd₂ such that 0.1 mm≤d₂≤0.5 mm, and length L₂ such that 0.05 mm≤L₂≤1.25mm, with the taper reducing the amount of liquid coating rejected by thedie. It has further been determined herein that the performance ofcone-only coating dies is also unexpectedly and materially influenced bythe coating chamber inner diameter and wetted length. The wetted lengthis defined as the distance between the exit of the guide die andentrance of the cone-only die. Disclosed herein are cone-only coatingdie designs with wetted length and coating chamber inner diametercharacteristics that result in increased stability and reduced strengthof the gyre, thereby resulting in a more stable and reliable coatingapplication process in manufacturing, particularly for applications atdraw speed≥50 mps (meters per second).

As fiber draw speeds increase or the diameter of the sizing diedecreases, more coating is rejected in the sizing die taper, and thegyre in the sizing die gets stronger. This leads to both increasedinstability of the coating process, which causes an increase in coatingoffset, and increased incidence of flooding, which causes fiber breaksand increased manufacturing costs.

In FIG. 5 is shown the schematic of a cone-only coating die coatingsystem, wherein cone-only coating die 50 is held by die holder 58 andhas a conical portion 52 having a cross-section with inner wall angledat a half angle α, cone height L₁, cylindrical portion 54 having innerdiameter d₂ and length L₂, with the wetted length in the coated chamber(defined as the distance between the exit 60 of the guide die andentrance of the cone die) as L₅ and inner radius of coating chamber 62has L₆. In FIG. 6 is shown the size and structure of gyre 25 fordifferent heights between the cone-only coating die 50 and the dieholder 58 (also defined as proud height, L₇). The difference in heightbetween the cone-only coating die 50 and the die holder 58 for the fourcases shown correspond to the wetted lengths listed in the table below:

Distance between Cone Die and Die Holder Wetted FIGS. (Proud Height), L₇(mils) Length, L₅ (mm) 6A, 7A 1 4.98 6B, 7B 23 4.42 6C, 7C 70 3.22 6D,7D 150 1.19

As can be seen, the size of the gyre is significantly reduced withreduction in the wetted length. The contours represented in FIGS. 6A-Dcorrespond to the gyre region in the coating die in which significantrecirculation of the coating fluid occurs. The gyre contributes to theflow instability in the coating die and the reduction of the gyreresults in a more stable and reliable fiber coating process. Thecalculations are shown for the case of coating chamber inner radius L₆of 8.7 mm. Suitable dimensions for coating chamber inner radius L₆ areabout 3 mm to 10 mm or about 7 mm to 9 mm. The corresponding positivecomponent of the radial velocity profile is shown in FIG. 7, with thesize and strength of the gyre significantly reduced with the reductionin wetted length. The lubrication pressure for the four cases shown inFIG. 8 is from about 460 psi to about 490 psi, with the lubricationpressure decreasing with decreasing wetted length. Lubrication pressureis defined as the fluid pressure at the transition point from theconical taper to the land region of the cone-only die. The lubricationpressure in a conventional design with similar dimensions, such as thedesign shown in FIG. 2, is about 566 psi, which is larger than thelubrication pressure for all the four cases shown in FIG. 8. In someembodiments, the lubrication pressure is less than 500 psi. In someother embodiments, the lubrication pressure is less than 450 psi. Themaximum temperature in the gyre for the four cases is shown in FIG. 9.The maximum temperature in the gyre decreases as the wetted lengthincreases. By controlling the wetted length, the maximum temperature inthe gyre can be controlled to a temperature less than or equal to 175°C., or less than or equal to 155° C., or less than or equal to 140° C.,or less than or equal to 130° C., or less than or equal to 115° C., orless than or equal to 100° C. In some embodiments, the maximumtemperature in the gyre is a temperature in the range from 70° C.-175°C., or in the range from 100° C.-150° C., or a temperature in the rangefrom 120° C.-140° C., or in the range from 120° C.-130° C. FIGS. 8 and 9show that control of wetted length permits simultaneous control oflubrication pressure and the maximum temperature of the gyre.Lubrication pressure and maximum gyre temperature can accordingly beoptimized for a particular coating material by controlling the wettedlength of the cone-only coating die. The coated diameter and shearstresses applied on the surface of the fiber in the cone-only die taperand land regions predicted for the different wetted length designs areshown in FIGS. 10 and 11, respectively. Results shown in FIGS. 6-10 arefor the following cone-only die design parameters: a=8°; L₁=0.7 mm;L₂=0.216 mm; d₂=0.216 mm and d₃=2.54 mm. The coated diameter refers tothe outer diameter of the coating on the fiber, where the fiber has adiameter of 125 μm.

Cone-only coating die systems are disclosed here having die designs thatincludes a conical ferrule with tapered wall, wherein the conicalferrule has a cross-section with inner walls angled at a half angle α,where 2°≤α≤25°, and cone height L₁ between 0.25 mm and 2 mm; acylindrical portion having an inner diameter of d₂ such that 0.1mm≤d₂≤0.5 mm, and length L₂ such that 0.05 mm≤L₂≤1.25 mm, wetted lengthL₅ such that 1 mm≤L₅≤5 mm (e.g., L₅<4.5 mm, or L₅<3.5 mm or L₅<1.5 mm,or 1 mm≤L₅≤4 mm, or 1 mm≤L₅≤3 mm, or 1.5 mm≤L₅≤4 mm) and coating chamberinner radius L₆ such that 3 mm≤L₆≤10 mm, or 3.5 mm≤L₆≤9.5 mm, or 4mm≤L₆≤9 mm, or 4.5 mm≤L₆≤8.5 mm, or 5 mm≤L₆≤10 mm, or 6 mm≤L₆≤9.5 mm or7 mm≤L₆≤9 mm.

In an embodiment, a process of coating an optical fiber during a fiberdraw process is disclosed wherein the optical fiber is drawn at a drawspeed V_(d) and the process of coating the optical fiber comprisespassing the drawn optical fiber through a guide die and thereafterthrough a cone-only coating die, the cone-only coating die having atemperature T_(die) and conical entrance portion with a tapered wallangled at a half angle α, wherein 2°≤α≤25°, and a cone height L₁ between0.25 and 2 mm, and a cylindrical portion having an inner diameter of d₂,wherein 0.1 mm≤d₂≤0.5 mm and a cylindrical height of L₂, wherein 0.05mm≤L₂≤1.25 mm, the guide die having an optical fiber exit, the guide diedisposed adjacent the cone-only coating die such that a wetted length L₅between the optical fiber exit of the guide die and the entrance of thecone-only coating die is from 1 mm to 5 mm, the cone-only coating dieand the guide die being held in a fixed relationship by a holder todefine a coating chamber between the guide die and the cone-only coatingdie, the coating chamber having an inner radius L₆ from the opticalfiber to an inner wall of the holder that is from 3 mm to 10 mm, thecoating chamber holding a liquid coating composition that is entrainedon a surface of the optical fiber as the optical fiber is drawn throughthe cone-only coating die resulting in a coating thickness of liquidcoating composition of d_(coat) on the surface of the optical fiber. Inone embodiment, the draw speed V_(d) is larger than 45 m/s. In anotherembodiment, the draw speed V_(d) is larger than 50 m/s. In anotherembodiment, the draw speed V_(d) is larger than 60 m/s. In someembodiments, the ratio of wetted length, L₅, to draw speed, V_(d), is inthe range from 0.01 msec to 0.15 msec. In other embodiments, the ratioof wetted length, L₅, to draw speed, V_(d), is in the range from 0.025msec to 0.125 msec.

In some embodiments, the cone-only coating die is a primary coating diewith the liquid coating composition as the primary coating composition.In these embodiments, the thickness of the primary coating compositionon the surface of the optical fiber, d_(coat), is between 10 microns and50 microns. In other embodiments, the cone-only coating die is asecondary coating die with the liquid coating composition as thesecondary coating composition. In these embodiments, the thickness ofthe secondary coating composition on the surface of the optical fiber,d_(coat), is between 10 microns and 50 microns. In some embodiments, thecone-only coating die is an ink coating die with the liquid coatingcomposition as the tertiary ink coating composition. In theseembodiments, the thickness of the tertiary ink coating composition onthe surface of the optical fiber, d_(coat), is between 2 microns and 10microns.

In some embodiments, the wall temperature of the cone-only coating die,T_(die), is between 25° C. to 75° C. In other embodiments, thetemperature of the cone-only coating die, T_(die), is between 40° C. to70° C. The fiber temperature entering the cone-only coating die ispreferably between 40° C. to 85° C. In some embodiments the coating isdelivered to the die at a temperature of greater than 35° C. In otherembodiments, the coating is delivered to the die at a temperature ofgreater than 45° C. In still other embodiments, the coating is deliveredto the die at a temperature of greater than 55° C.

The liquid coating compositions are preferably UV curable acrylatecompositions. In some embodiments, the liquid coating compositionentrained on the surface of the optical fiber is cured downstream usinga light emitting diode (LED) UV source having an emission spectrum witha peak wavelength in the range between 300 nm-450 nm.

The described embodiments are preferred and/or illustrated, but are notlimiting. Various modifications are considered within the purview andscope of the appended claims.

What is claimed is:
 1. A process of coating an optical fiber during afiber draw process wherein the optical fiber is drawn at a draw speedV_(d), and the process of coating the optical fiber, comprises: passingthe drawn optical fiber through a guide die and thereafter through acone-only coating die, the cone-only coating die having a walltemperature T_(die) and a conical entrance with a tapered wall angled ata half angle α, wherein 2°≤α≤25°, and a cone height L₁ less than 2.2 mm,and a cylindrical portion having an inner diameter of d₂, wherein0.1≤d₂≤0.5 mm, and a cylindrical height of L₂, wherein 0.05≤L₂≤1.25 mm,the guide die having an optical fiber exit, the guide die disposedadjacent the cone-only coating die such that a wetted length L₅ betweenthe optical fiber exit of the guide die and the conical entrance of thecone-only coating die is less than 4.5 mm, the cone-only coating die andthe guide die being held in a fixed relationship by a holder to define acoating chamber between the guide die and the cone-only coating die, thecoating chamber having an inner radius L₆ from the optical fiber to aninner wall of the holder that is from 3 mm to 10 mm, the coating chamberholding a liquid coating composition that is entrained on a surface ofthe optical fiber as it is drawn through the cone-only coating dieresulting in coating thickness of d_(coat) on the surface of the opticalfiber.
 2. The process of claim 1, wherein the wetted length L₅ is lessthan 3.5 mm.
 3. The process of claim 1, wherein the inner radius L₆ ofthe coating chamber is from 6 mm to 10 mm.
 4. The process of claim 1,wherein the cone height L₁ is between 0.25 mm and 2.0 mm.
 5. The processof claim 1, wherein the draw speed V_(d) is larger than 45 m/s.
 6. Theprocess of claim 1, wherein the draw speed V_(d) is larger than 60 m/s.7. The process of claim 1, wherein a ratio of wetted length L₅ to drawspeed V_(d) is between 0.01 msec to 0.15 msec.
 8. The process of claim7, wherein the ratio of wetted length L₅ to draw speed V_(d) is between0.025 msec to 0.125 msec.
 9. The process of claim 1, wherein thethickness of the liquid coating composition on the surface of theoptical fiber d_(coat) is between 2 microns and 50 microns.
 10. Theprocess of claim 1, wherein the liquid coating composition that isentrained on a surface of the optical fiber is a UV curable acrylatecomposition and is cured using a light emitting diode UV source havingan emission spectrum with a peak wavelength in the range between 300nm-450 nm.
 11. The process of claim 1, wherein the cone-only coating dieis a primary coating die and the liquid coating composition that isentrained on a surface of the optical fiber is a primary coating and thethickness of the liquid primary coating composition on the surface ofthe optical fiber d_(coat) is between 10 microns and 50 microns.
 12. Theprocess of claim 1, wherein the cone-only coating die is a secondarycoating die and the liquid coating composition that is entrained on asurface of the optical fiber is a secondary coating and the thickness ofthe liquid secondary coating composition on the surface of the opticalfiber d_(coat) is between 10 microns and 50 microns.
 13. The process ofclaim 1, wherein the cone-only coating die has a lubrication pressurethat is less than 500 psi.
 14. The process of claim 1, wherein a gyre isformed in the coating chamber having a maximum temperature of less than175° C.
 15. The process of claim 14, wherein the maximum temperature ofthe gyre is less than 100° C.